1. Field of the Invention
The present invention relates to a rotation angle detecting device that detects the rotation angle of a rotary shaft of a rotating object, such as a steering wheel of a vehicle, by detecting change of magnetic flux density.
2. Description of the Related Art
JP-A-2005-3625 or U.S. Pat. No. 6,861,837 B1, which is a counterpart of the former, discloses a prior art rotation angle detecting device that detects a rotation angle of a rotating object larger than 360 degrees in angle. As shown in FIG. 14 of this application, the prior art rotation angle detecting device includes a pair of permanent magnets 80, 90 each of which is separately linked with a rotary shaft 40 of a rotating object via gear mechanism and a pair of magnetic sensors 100, 110, each of which detects magnetic flux density of a magnetic field generated by the permanent magnets 80, 90. The gear mechanism is comprised of three gears: a drive gear 70 that is fixed to the rotary shaft of the rotating object and two driven gears 50, 60 to which the permanent magnets 80, 90 are respectively fixed. The number of teeth of the driven gear 50 is different from the other driven gear 60 to change the phase between the output signals of the magnetic sensors 100, 110 as the driven gears 50, 60 rotate so that the rotation angle larger than 360 degrees in angle can be calculated from the phase difference. In other words, the gear mechanism includes at least three gears (70, 80, 90), which increase the size and parts of the rotation angle detecting device.